In an optical transmission system a loss of input signal alarm is an important tool for determining that an optical cable has been broken, removed, or introduces a high loss.
Improvements in optical transmission methods have vastly enhanced the use of the optical communication systems by increasing both the data rates and the distance over which optical signals are transmitted. Erbium doped optical amplifier (EDFA), one of the latest components in photonic systems, replaces the regenerator (repeater) in many applications. An optical amplifier can amplify optical signals without optically demultiplexing them, thereby avoiding the costs of multiple optical receivers, multiple regeneration circuits and multiple optical transmitters. One of the major advantages of optical amplifiers is that they amplify whatever bit rate comes down the fiber. Even if the transmission rate is boosted, the device will not need to be replaced.
On the other hand, there are no error counts possible between optical amplifiers and therefore isolating the cause of a degraded error rate is not a simple task. Consequently, there is a need to provide a method and apparatus for troubleshooting a chain of optical line amplifiers, where no parity error counts are available.
In optical amplified systems, the reflection of a significant portion of the light leaving via a given fiber may cause problems with detection of the loss of the input signal on that fiber. If the reflected outgoing light could be distinguished from the desired input signal, then appropriate alarms or control actions could be initiated. The outgoing light, that is then reflected, could be amplified signal and amplified spontaneous emission (ASE), as in the case of a bidirectional system, or could be just ASE, as in the case of a unidirectional system. Or, the outgoing light could be a combination of signals and ASE from both directions in the case where there are more complex optical path reflections.
Especially in bi-directional optical amplifier applications, reflections can cause an optical amplifier to oscillate despite the optical isolators that may be present. This oscillation path can involve more than one optical amplifier in the system and be quite complex.
Measurement of the strength of reflections is presently done with an optical time domain reflectometer (OTDR) that sends strong short pulses of light down a fiber and measures the signal returned. This is an accurate method, but the OTDR is a relatively large and expensive piece of test equipment that can not easily be used while there is traffic on the fiber.
Optical frequency domain reflectometry may also be used to detect faults in an optical link. According to this method, the optical frequency is varied and optically coherent detection is used (IEEE Photonics Technology Letters, Vol. 2, No. 12, pp. 902-904, December 1990), or an optical source is modulated with a constant amplitude tone that is swept in frequency (Applied Optics, vol. 20 no 10, pp. 1840-1844, 1981).
Another method for detecting a fault in an optically amplified system is to use a correlation of a specifically generated pseudo-random pulse sequence for reducing peak power requirements (Applied Optics, Vol. 22, No. 23, pp. 3680-3681, 1983).
Still another prior art method is to measure the amount of DC light reflected back via a four port coupler. However, this method does not stimulate or consider the AC portion of the signals. The DC reflection is used to determine if a large reflection from a broken fiber or open connector is present so as to shut-down the output of the optical amplifier for safety. In addition, this has been known to falsely trigger from low level reflection due to Raleigh scattering in the fiber. This method cannot be used in bidirectional systems.
There is a need to provide a means for detecting errors in a transmission system irrespective if a data signal is present or absent. There is also a need to distinguish optical reflections from valid inputs when isolating a cable break in an optically amplified system.